Answers

That doesn't look good does it, it's most likey under or over watered and is dying because of it.

I have one planted in my front garden and in my experience we don't get enough hot sun to let them flower when they're outside - they certainly flourish, but don't flower - they produce loads of flower stems and buds, but there isn't the enough hot sun to ripen them.

Yours though look like they're on their way out and i'd get them out of those root killing pots and let them loose into the ground where they're asking to be.

Louise1 and Beattie
Many thanks for your quick replies.
The supplier said there is a lifetime guarantee, and no mention of de-potting them or frost being a problem.
However I agree there's no getting away from the comments about the sunshine!

The only variety of Oleander that I would even try outdoors in the UK is 'Little Red', and it tends to bloom only once in the spring. On top of that, the winter you guys have just had would test even that variety's frost hardiness. They will bloom even in 50% shade here, so even your version of "full sun" should still be enough to trigger bloom. Heat gain is another story, and outdoors, I would put it on the south side of a stone wall to be sure. In pots, they would need more water than you would think: here, we water twice a day at the summer, even in heavy compost, just to keep them from wilting. The difference in temperature would probably adjust that to about once every day or two in the UK.

Beattie's right, these plants don't like extreme cold weather and need a bit of protection - this explains why they look so awful now. They do, as already said, require a lot of sun, so planted (or stood) in a southfacing situation, with shelter behind and perhaps around, they will flower well. As a point of interest, I have one in the ground in a client's front garden - south facing, lots of shelter around in the form of house wall, path, low brick wall in front, and it's got buds on it right now I noticed last week. All those hard surfaces in a south facing garden give off heat after the sun's been on them, so provide good shelter for this kind of plant.